This invention relates to a Bloch line memory device and a method for operating same and in particular to a Bloch line memory device and a method for operating same, which are suitable for obtaining a good read-out margin and practical.
In a Bloch line memory device a magnetic garnet film is used as a memory medium film as in a magnetic bubble memory device. However their memory methods are considerably different. That is, in a Bloch line memory the presence and the absence of a vertical Bloch line pair existing in the wall around a stripe magnetic domain obtained by stretching a bubble correspond to "1" and "0", respectively, while in a prior art magnetic bubble memory the presence and the absence of a bubble correspond to "1" and "0" in data. FIG. 1 indicates this aspect. In the figure, the arrow directed upward in a stripe magnetic domain 2 shows the direction of magnetization; arrows 101 on the center line of the wall 1 show the direction of magnetization located at the wall center; and arrows 102 perpendicular to the wall 1 at the center line show the direction of magnetization at the center of the vertical Bloch line (hereinbelow called merely Bloch line). Further, portions, where a pair of Bloch lines exist, correspond to "1" in data and portions, where no Bloch lines exist, correspond to "0".
The Bloch line used as an information carrier is a microstructure of the domain wall existing in the wall 1 surrounding the magnetic domain. The Bloch line can exist stably in the domain wall and propagate freely along the domain wall. Consequently, when a number of stripe magnetic domains are arranged at their predetermined positions and Bloch lines are made to exist in the domain wall, they behave just as bubbles propagating in a minor loop of a magnetic bubble memory. For this reason a Bloch line memory is a shift register type memory similarly to a magnetic bubble memory.
The existence of the Bloch line is known since long ago and it is verified by experiments and their analysis that the mobility of the magnetic domain is reduced by the existence of the Bloch line. Consequently, for the magnetic bubble memory, for which the magnetic domain should be shifted, the bubble domain containing Bloch lines is called a hard bubble and attempts have been made to prevent its generation. To the contrary, for the Bloch line memory device, the existence of this Bloch line is positively utilized.
The physical size of the Bloch line is about 1/10 of the width of the stripe magnetic domain, where the Bloch line exists, and a number of Bloch lines can exist in one stripe magnetic domain. For example, at present, in a garnet film having stripe magnetic domains 1 .mu.m wide developed for a magnetic bubble memory it is possible to make about 5.times.10.sup.6 Bloch lines exist per 1 cm.sup.3. Therefore, in the case where 2 Bloch lines are paired in the information medium, it is possible to realize a memory of 256M bit/cm.sup.2.
Furthermore there is another reason why a Bloch line memory device can have a large memory capacity, apart from the fact that the size is extremely small. It is due to the fact that in the Bloch line memory the magnetic field in the vertical direction is used for the propagation of information, while in the magnetic bubble memory information carriers are propagated by rotating an in-plane field. For this reason there is a high possibility that the propagation pattern is simplified in a plane, which acts so as to make it easier to increase the density for this type of memory elements.
As described above, vertical Bloch lines in the garnet film are movable in the domain wall and can work as information carriers. However, in order to realize a memory element, it is necessary to write-in and read-out information, if necessary. The write-in operation is effected basically by making electric current flow through a conductor disposed at the proximity of one stripe domain head so that a local magnetic field is generated there and reversing the direction of magnetization in the domain wall. That is, it can be thought that the direction of magnetization indicated by "0" in FIG. 1 is reversed so that it is in accordance with the direction of magnetization in the "1" domain. At this time, at the boundary between the domain wall, where the direction of magnetization is reversed and the domain wall, where it is not reversed, the direction of magnetization varies continuously and a state where the magnetization has changed by 90.degree. with respect to the domain wall is established. Bloch lines are produced in this way. Further two Bloch lines are always paired. Consequently the Bloch line memory is constituted by making one pair of Bloch lines correspond to one information.
The read-out of information is effected, after having transformed the presence or absence of the Bloch line into the presence or absence of the bubble domain. The transformation of the Bloch line into the bubble domain is effected by the method described by Konishi in an article published in IEEE Trans. Mag. 19, No. 5 (1983) p. 1838-p. 1840 and p. 1841-p. 1843. That is, when a pair of Bloch lines exist, the direction of magnetization in the domain wall is reversed at the pair of Bloch lines as a border, as indicated in FIG. 1. When a pair of Bloch lines arrive at the stripe magnetic domain head, variations in this magnetization structure give rise to variations in chopping properties. Therefore, when a proper chopping condition is selected, it is possible to chop a bubble 8, only when a pair of Bloch lines exist at the stripe magnetic domain head. When the chopped bubble 8 is propagated to a detector as in the major line of a bubble memory and transformed into electric signals, the existence of the pair of Bloch lines can be read-out. FIG. 2 is a top view of the element indicated in FIG. 1, in which also the direction of magnetization seen from above is indicated by arrows.
The principle of the transformation of the pair of Bloch lines into a magnetic bubble domain will be explained in detail, referring to FIGS. 2A and 2B. When a predetermined electric current is made to flow through a hair-pin shaped conductor 7 superposed on the stripe magnetic domain 2, as indicated in FIG. 2A, so that a magnetic field is applied to the gap of the conductor 7 in the collapse direction, the domain walls 1 at both the sides of the stripe magnetic domain approach each other. Noticing the magnetizations 5 of the domain walls, which have approached each other, it can be seen that the magnetizations have a same direction at the upper and the lower side of the domain wall, because the magnetizations are inverted to each other at the boundary formed by the pair of Bloch lines. For this reason influences of the exchange interaction acting on the magnetizations 5 are small and the domain walls are combined by a weak magnetic field in the collapse direction. As the result a new magnetic bubble domain 8 is generated. In this case the chopping of the Bloch lines is easy.
To the contrary, in the case indicated in FIG. 2B, where no Bloch lines exist, the magnetizations 5 are not reversed because of the absence of the Bloch lines and thus the magnetizations are opposite to each other at the upper and the lower side of the domain wall. Therefore, even if a chopping operation is effected, influences of the exchange interaction acting on the magnetizations are remarkable and thus it is difficult to combine the sides of the domain wall. For this reason no magnetic bubble domain is chopped, i.e. the chopping of the Bloch lines is difficult.
In order to detect whether the chopping of the Bloch lines described above is easy or difficult, it is necessary to set the intensity of the magnetic field in the collapse direction in a certain region. That is, if the magnetic field is too strong, the head portion of the stripe magnetic domain is chopped and to the contrary, if it is too weak, no magnetic domain is chopped, even if a pair of Bloch lines exist at the head portion. Consequently the intensity of the magnetic field should be selected in this region. For this selection it is sufficient to select an intensity of the electric current. A concrete example is shown here. For a material having stripe magnetic domains 5 .mu.m wide, supposing that the gap between the conductors is 5 .mu.m, when the intensity of the electric current is in a region between about 120 and 150 mA, the presence or absence of the Bloch lines can be transformed into the presence or absence of the bubble.
It is possible to realize a Bloch line memory by forming a write-in, a memory and a read-out function portion described above in one element.
As described above, an information in the Bloch line memory is represented by the presence or absence of a pair of Bloch lines. Consequently, in a practical memory device it is necessary to transform the presence or absence of the pair of Bloch lines into the presence or absence of the bubble. FIG. 3A indicates a case where a pair of Bloch lines 4 arrive at the head portion of a stripe magnetic domain 2 and on the other hand FIG. 3B indicates a case where there are no pair of Bloch lines. In the two states, comparing the directions of the magnetizations 5 of the domain wall 1, it can be seen that the two are the same. That is, it can be understood that the direction of the magnetization of the domain wall varies, depending on whether Bloch lines exist or not, but the direction of the magnetization doesn't vary, depending on whether a pair of Bloch lines exist or not. This reason is that the direction of the magnetization reversed by one Bloch line is reversed again by the other Bloch line so that the direction of the magnetization returns finally to the initial one. For this reason, since the direction of the magnetizations 5 doesn't vary depending on the presence or absence of the pair of Bloch lines, it can be understood that its presence or absence cannot be transformed into the presence or absence of the magnetic bubble domain by the prior art method utilizing only a hair-pin shaped conductor 7 in FIG. 2.
As measures for resolving this problem a method is disclosed in JP-A-59-101092. By this method electric current is made to flow through a conductor superposed on the chopping conductor so that the position of the pair of Bloch lines is held and reading-out is carried out by effecting the chopping operation between the pair of Bloch lines. According to this method the presence or absence of the pair of Bloch lines can be transformed into the presence or absence of the bubble domain and thus the reading-out according to the prior art techniques is realized. However it is not possible to obtain any satisfactory working margin only by this method. That is, since the distance between the pair of Bloch lines is about 0.2 time as large as the width of the stripe magnetic domain, it is almost impossible to dispose the chopping conductor therebetween.
Further, since the pair of Bloch lines existing on the side surface of the stripe magnetic domain are read-out by this method, there are two read-out positions on the sides of th domain wall, which are opposite to each other. In addition, this method has another disadvantage that when the magnetic domain is chopped at the reading-out, the information existing at the head portion of the stripe magnetic domain is lost.
As another article on the Bloch line memory device, refer to Electronic Product Design, Oct. 1985, p. 69-p. 72.